1. Field of the Invention
This invention relates to a process for preparing aryl halides and in particular to a process for selectively substituting an aromatic nitro group with a halogen.
2. Related Information
A common problem facing organic chemists is the regio-specific halogenation of aromatic rings. One practical solution to this problem is the substitution of aromatic nitro groups by a halogen, e.g., chlorine. The Sandmeyer reaction is normally used to accomplish this conversion. The nitro group is reduced to an amine, diazotized, then reacted with copper chloride to give the corresponding chloroaromatic. A number of variations on this basic reaction are also known. Other methods of replacing an aromatic nitro group with a chlorine include irradiation in chloroform/hydrogen chloride solution, alkylative reduction by Grignard reactions quenched with sodium hypochlorite and treatment with thionylchloride in the vapor phase.
The Sandmeyer reaction is normally the process of choice and even though this synthesis involves many steps, it offers several advantages over direct halogenation. First of all, fluorides and iodides which can seldom be prepared by direct halogenation, can be obtained from the diazonium salts. Second, where direct halogenation yields a mixture of ortho and para isomers, the ortho isomer, at least, is difficult to obtain pure. On the other hand, orthod and para isomers of the corresponding nitro compounds, from which the diazonium salts ultimately come, can often be separated by fractional distillation. For example, the boiling points of ortho- and para-bromotoluenes are only 3.degree. C. apart (82.degree. C. and 85.degree. C.). The boiling points of the corresponding ortho- and para-nitrotoluenes, however, are 16.degree. C. apart (222.degree. C. and 238.degree. C., respectively).
The use of phosphorushalides of formula: R.sub.n PX.sub.5-n wherein n is selected from 0, 1, 2 and 3; R is selected (when n is other than 0) from the group consisting of C-6 to C-10 aryl and substituted aryl wherein the substituents are selected from the group consisting of: straight and branched chain alkyl, alkoxy, and haloalkyl; halogen, sulfonate and mixtures thereof; and X is a halogen, and particularly the use of phenylphosphorustetrachloride (PPTC), as a reagent for organic synthesis is practically unknown when they contain aryl or substituted aryl groups. Timokhin, B. V.; Dmitriev, V. K., Dmitriev, V. I., Zh. Obshch. Khim 1984, 54, 1290, reported the reaction of cyclohexene with PPTC to give trans-1,2-dichlorocyclohexane and 3-chlorocyclohexene. Mitrasov, Y. N.; Vladyko, E. D.; Kormachev, V. V. USSR SU Nos. 1,051,097 and 1,051,096 found that treatment of aliphatic aldehydes and ketones with PPTC produced geminal dichlorides. PPTC has also been used to produce tetrazines from hydrazines, see Yagupol'skii, L. M.; Matyushecheva, G. I.; Mikhailov, V. S.; Bulygina, L. A. USSR SU No. 498,300 and Matyushecheva, G. I.; Mikhailov, V. S. Yagupol'skii, L. M., Zh. Org. Khim, 1974, 10, 124.